SUMMARY

SUMMARYThe purpose of the present monograph is to give an account of the distribution of fibrinolytic components in the organism, with special reference to the tissue activator of plasminogen.Fibrinolytic activity of tissues.–Various tissues from man and animals can digest fibrin. It has previously been shown that this digestion is referable to an activator present in the tissues; this activator is capable of transforming the fibrinolytic pro‐enzyme, plasminogen, which is present in fibrin, into the active fibrinolytic enzyme, plasmin.Several tissues also contain components with an inhibitory effect on the fibrinolytic process. The nature of most of these components is unknown. However, it is known that a low‐molecular plasmin inhibitor (pulmin) is present in tissues from the bovine lung and uterus, and it has been shown that no other tissues contain pulmin.Great difficulties have been encountered in elaborating methods for qualitative and quantitative determinations of the concentration of the tissue activator in fresh tissues, particularly because the tissue activator is firmly bound to the tissue proteins. A review of previously used analytic methods shows that they all had some qualitative or quantitative shortcomings. The elaboration of an analytic method for the quantitative determination of the content of plasminogen activators in the tissues was therefore a prerequisite for the present study. Based on the observation that potassium thiocyanate is a specific solvent for the extraction of the tissue activator it was possible to elaborate such a method, by which the concentration of the tissue activator could be expressed in terms of arbitrary units. The tissue concentration of pulmin may also be determined by this method.The plasminogen activator of the tissues is relatively stable and differs in that respect from the plasminogen activator present in the blood under certain conditions. Thus, it is not destroyed by heating to 70 or 100° C. at acid reaction, and it is stable at 37 and 50° C. within a very wide pH range. It is also stable to the action of a large number of chemicals, but is destroyed by formalin. It is very firmly bound to the tissue proteins and can be extracted only by repeated treatments with potassium thiocyanate. The activator is presumably chiefly localised in the microsomes.A number of studies have shown that the tissue activator presumably reacts stoicheiometrically with plasminogen in an equilibrium process.Quantitative determinations of the content of tissue activator have revealed considerable variations from organ to organ in the same animal, from animal to animal of the same species, and from one animal species to another. These variations are at present unexplained.Wide variations in the concentration of the tissue activator have also been revealed in various human tissues, but it is possible to classify the organs into groups with high, moderate and low concentrations. The first group comprises such organs as the uterus, adrenals, lymph nodes, thyroid gland, lungs, prostate and ovaries, whereas liver tissue is practically inactive.The significance of the tissue activator is not definitely known. It has often been emphasised that it should be of the greatest importance in the redissolution of fibrin deposits and thus be able to counteract the formation of connective tissue. It has further been claimed that a relationship exists between the tissue activator and the occurrence of local bleedings in the tissues; by causing a redissolution of the fibrin deposits formed for the purpose of haemostasis it should be a contributory factor in the persistence of the bleeding. It is unknown whether or not the tissue activator is of importance in the fibrinolytic activity of the blood in vivo or post mortem. Some experimental data are in favour of the assumption that the tissue activator under certain conditions (tissue destruction, cell necrosis) may be liberated from the cells and enter the circulating blood.Fibrinolytic activity of the uterus.–Both human and animal myometrial tissues contain a plasminogen activator. Thus, the myometrium is the tissue of the human organism which contains the activator in the highest concentration.Plasminogen activators are also present in the human endometrium. The concentration varies with the various stages of the menstrual cycle; it is only moderate in the proliferative and intermediate stages, but considerable in the secretory stage. The concentration decreases with age; the senile endometrium does not contain tissue activator. Considerable amounts of tissue activator are present in certain pathological endometria (endometrial hyperplasia, etc.). It is reasonable to assume that a relationship exists between the tissue activator of the endometrium and the female sex hormones. Such a relationship has also been observed in animal experiments, in which it has been shown that a decrease in the blood concentration of oestrogens results in an increase in the uterine concentration of the tissue activator.Based on these observations the theory has been advanced that the tissue activator is of significance in the occurrence of uterine bleedings. It has thus been thought that the decreasing oestrogen concentration in the circulating blood in the premenstrual phase through a constriction of the spiral artery should give rise to incipient cell necrosis in the endometrium and thus provide optimum conditions for a liberation of the tissue activator. This liberation should be a contributory factor in the occurrence of bleeding from the tissue.The tissue activator is absent in normal decidual and placental tissues, but present in pathological decidual tissue from spontaneous abortions. It is possible that the tissue activator in this pathological tissue may be a contributory factor in the occurrence of bleeding from the tissue and thus be of importance in the development of spontaneous abortion.Normal menstrual blood does not clot spontaneously or after addition of thrombin. This is due to the absence of fibrinogen, because the menstrual blood has previously been clotted in the uterine cavity. This coagulation process is presumably due to a liberation of thromboplastin from the endometrium. Subsequent redissolution of the fibrin clot formed in this way is caused by a fibrinolytic process excited by the liberation of the tissue activator from the endometrium. This has been shown by the finding of a stable plasminogen activator of the tissue‐activator type in menstrual blood and by the presence of plasmin and the absence of plasminogen. Thus, the incoagulability of menstrual blood is due to the combined effects of tissue thromboplastin and tissue activator, which have both been liberated from the injured endometrial tissue.As distinct from the local afibrinogenaemia of menstrual blood, hypo‐ or afibrinogenaemia has often been observed in the circulating blood, especially as an obstetrical complication. It is not known with certainty whether this condition may also be due to a liberation of tissue components (thromboplastin, tissue activator) from the uterus.Fibrinolytic activity of the prostate and seminal fluid.–Prostatic tissue is fibrinolytically active. It has been shown that this activity is due to the presence of two plasminogen activators in the tissue, of which one corresponds to the stable plasminogen activator of the tissues and the other to the labile plasminogen activator of the blood.Seminal fluid likewise contains plasminogen activators. It has been shown that the plasminogen activator of seminal fluid corresponds to the plasminogen activator of the tissues and is stable, for which reason it presumably originates from the prostate. However, the experimental results available do not exclude that the seminal fluid also contains plasminogen activators of the labile type.Whereas the plasminogen activators of the prostate are thus presumably of importance for the fibrinolytic activity of seminal fluid, it is not yet known with certainty whether they may also give rise to the fibrinolytic activity in the circulating blood which has been observed in cases of cancer of the prostate and after prostatectomy.Fibrinolytic activity of secretions and transudates. –Previous investigations on the fibrinolytic enzyme system of the blood have now rendered it possible to subject the fibrinolytic systems of other body fluids to a closer analysis.Some secretions (milk, tears and saliva) contain fibrinolytic enzyme systems which differ from that of the blood. They are characterised by the presence of variable amounts of plasminogen activators and large amounts of plasminogen pro‐activators which can be transformed into the activator itself by addition of streptokinase. As distinct from blood, the secretions do not contain trypsin inhibitors. On the basis of these results it is discussed whether the fibrinolytic components originate from the blood or are secretory products from the gland concerned. The former assumption appears to be more likely. Accordingly, the glandular epithelium must be assumed to be selectively permeable to the active components, but capable of retaining the inhibitors.A number of normal and pathological transudates also contain fibrinolytic enzyme systems largely corresponding to that of the blood. Thus, they contain large amounts of trypsin inhibitors and plasminogen pro‐activators. However, cerebrospinal fluid differs from the other transudates in that it does not contain inhibitors and is thus reminiscent of the secretions. In the present study, special attention was focused on the fibrinolytic enzyme system of amniotic fluid because it may be of significance in the haemorrhagic diatheses which have occasionally been observed in relation to amniotic‐fluid infusions.